Bertec Force Plate Setup

This page provides instructions on how to integrate a Bertec force plate system with an OptiTrack motion capture system.

Overview

When a motion capture system is used in conjunction with force plates, they work together as an efficient tool for various research applications including biomechanical analysis, clinical gait analysis, physiology research, sports performance research, and many more. An OptiTrack motion capture system can synchronize with force plates to obtain both kinematic and kinetic measurements. Note that force plate integration is supported only with a Prime camera system using the eSync synchronization hub. This page provides quick guidelines for setting up and configuring force plates — with digital outputs — along with the OptiTrack motion capture system.

For detailed information on specifications and configurations on the force plates, refer to the documentation provided by the force plate manufacturer.

Analog Platforms

Analog force plate devices can only be implemented via DAQ devices. Incoming voltage signals can be detected through the data acquisition channels, but force plate related software features (vectors, position calibration, etc.) will not be supported in Motive for the analog platforms. Refer to the NI-DAQ Setup page for detailed instructions on integrating analog devices.

Requirements

Supported Amplifier Models: AM6800

Firmware Version: For synchronization support, the Bertec amplifiers must be installed with firmware version 4.5.2 or above. The current firmware version gets displayed on the 7-segment display when first powering up the amplifier. Please check this and make sure the firmware is updated to the supported versions.

Prime series Ethernet camera system with the eSync synchronization hub.

In order to integrate force plate systems with Motive, you will need to setup the required drivers and plugins. Motive installer is packaged with the Peripheral Device module which can be added. During the Motive installation, a list of program features will be shown in the Custom Setup section. Here, change the setting for the Peripheral Device module, as shown in the below image, so that the module is installed along with Motive Files.

Note : Even if you are not using NI-DAQ, it is still necessary to install NI-DAQmx drivers that come up next in the installer.

In addition to the Peripheral Device module, you may also want to install the Digital Acquire™ from Bertec to verify that the force plates are properly working. Visit the below webpage to download the software, and follow the respective instructions to install. This software installs remaining resources for connecting the Bertec force plates.

If the hardware and software for the force plates are configured and successfully recognized, Motive will list out the detected force plates with number labels (1, 2, etc..). Motive will notify you of incorrect or nonexistent force plate calibration files. When the devices are successfully instantiated in Motive, the Status Log will indicate that the device has been created and loaded.

Calibrate the capture volume as normal to get the orientation of the cameras (see the Quick Start Guide or Calibration page for more information). The position of the force plate is about the center of the volume, and when you recalibrate or reset the ground plane, you will need to also realign the position of your force plates for best results.

On the CS-400 calibration square, pull the force plate alignment tabs out and put the force plate leveling jigs at the bottom. The leveling jigs align the calibration square to the surface of your force plate. The alignment tabs allow you to put the CS-400 flush against the sides of your force plate giving the most accurate alignment.

Place the calibration wand on the force plate so that vertex of the wand is located at the right-hand corner of the side where the cable input is located (as shown in the image below). A correct placement of the calibration square is important because it determines the orientation of the force plate and its local coordinate axis within the global system. The coordinate systems for force plates are independent of the system used Motive.

Bertec force plates use the right-hand system. The longer arm of CS-400 will define the Y axis, and the shorter arm will define the X axis of the force plate. Accordingly, Z axis is directed downwards for measuring the vertical force.

Bertec force plate with CS-400 aligned properly.

Calibrated force plate position and orientation. X and Y axis is shown.

After placing the calibration square on the force plate, select the CS-400 markers in Motive. Right click on the force plate you want to locate, and click Set Position. When there are multiple force plates in a volume, you may need to step on the force plate to find which platform the calibration square is on. In Motive, uncalibrated force plates will light up in green and a force vector will appear when you step on the plate. Repeat step 4 and 5 for other force plates as necessary.

Referencing to the markers on the calibration square, Motive defines the location of the force plate axis within the global coordinate system. When setting the position, the previously defined force plate dimension values and mechanical-to-electrical center offset values will be applied and reflected in the Perspective View pane.

Tip: To double check that the dimensions are modified properly, you can place extra retroreflective markers on each corner of the platform and monitor the coincidence of the markers position with the force plate assets from the perspective view.

Setting the position of a force plate in Motive. The number label on the force plate is inverted because the force plate position and orientation has not been calibrated yet.

If the force plate dimensions are not automatically configured, you need to enter the dimensions of the force plate in the force plate properties after calibrating its positions. Go to the Devices pane and select the force plate, and its properties will get listed under the Properties pane. Enter the length and width (in meters) values for the corresponding plates as reported in the specifications.

After you have calibrated each of your force plates, remove the CS-400 from the volume. Right click one of your force plates in Motive and click Zero (all). This will tare the scale and set the current force on the plate data to 0. This will account for a small constant amount of measurement offset from the force plate. Remember that it zeros all of the force plates at once. So make sure there are no objects on any of the force plates.

Note: Zeroed scales of Bertec force plates are saved within their software driver, and each time the driver restarts, these settings are refreshed. This means that the force plate zero setting will be refreshed each time you start Motive, or each time the device is disabled and enabled back again in Motive. Please be aware of this behavior and zero your plates when necessary. In Motive, there is a Zero On Enable property setting for Bertec force plates under the Devices pane, and enabling this setting will automatically zero your plate each time the device is enabled or when Motive restarts. The Zero On Enable setting is enabled by default.

The sampling rate of force plates is configured through the synchronization setup, which will be covered in the next section. The eSync is required in order to synchronize the two systems together. It will be configured to send out either the reference clock signal or the recording trigger signal to the force plate amplifier. Depending on which type of signal is used for the synchronization, supported sampling rates may vary.

Supported Force Plate Sampling Rates

Reference Clock Signal Sync: When using the reference clock signal from the eSync to synchronize the force plates, the force plate will basically run at the same rate as the received clock signal. You can also apply either the multiplier or the divider to the outputted clock signal to make additional adjustments.

Recording Trigger Sync: When using the recording signal from the eSync to trigger-sync the force plates, the force plates will be running at their own free run sampling rate. In this case, only 1000 Hz sampling rate is supported, and you will need to adjust the camera frame rate in the Devices pane and apply framerate-multipliers to set the 1000 Hz sampling rate on the force plates.

There are two synchronization approaches you could take: Synchronization through clock signal or through recording trigger signal.

Synchronization via clock signal utilizes the internal clock signal of the eSync to synchronize the sampling of the force plates on per-frame basis. However, when there is another device (e.g. NI-DAQ) being synchronized to the clock signal frequency, the sampling rate cannot be set for each individual device. In that case, triggered sync must be used for synchronizing the initial recording trigger. Synchronization via trigger signal utilizes the recording trigger in Motive to align the initial samples from both systems. After the initial sync, both systems run freely at their own sampling rate. If the force plates are running at whole multiples of the camera system, the collected samples will be aligned. However, since the sampling clocks are not perfectly accurate, alignment of the samples may slowly drift over time. Thus, when synchronizing via recording trigger, it is better to keep the record times short.

When synchronizing through the eSync, use the following steps to configure the sync settings in Motive. This will allow both systems to be triggered simultaneously with reference to the master synchronization device, the eSync.

IMPORTANT NOTE: For this synchronization setup to work properly, the Bertec amplifier firmware must be updated to firmware version 4.5.2 or above. Currently installed firmware version gets displayed on the 7-segment display when first powering up the amplifier. Please check this and make sure the firmware is updated to the supported versions. If an older version is installed, please contact Bertec for instructions on updating the firmware.

In the Devices pane, select the eSync among the listed devices. This will list out the synchronization settings in the properties pane for the selected eSync.

In the Properties pane, under Sync Input Settings section, set the Source to Internal Clock.

Next, to the Clock Frequency section, input the sampling rate that you wish the run the force plates in. This clock signal will be eventually outputted to the force plate system to control the sampling rate. For this guide, let's set this to 1000 Hz.

Once the clock frequency is set, apply the Input Divider/Multiplier to the clock frequency to set the framerate of the camera system. For example, if you input 10 to the Input Divider section with internal clock frequency running at 1000 Hz, the camera system will be running at 100FPS. The resulting frame rate of the camera system will be displayed in the Camera Rate section.

Next step is to configure the output signal so that the clock signal can be sent to the force plate system. Under the Outputs section, enable the corresponding output port of the eSync which the force plate system is connected to.

Now that the eSync has been configured, you need to configure the force plate properties in Motive. While the force plate(s) is selected in Motive, access the Properties pane to view the force plate properties. Here, set the following properties:

Record Trigger → False

Reference Clock Sync → True

eSync Output Channel → output port used on the eSync.

Once this is set, the force plate system will start sampling at the frequency of the clock signal configured on the eSync, and this rate will be displayed on the Devices pane as well.

Example eSync properties for clock sync. Make sure the eSync is selected in the Properties pane.

Example force plate properties for clock sync. Make sure the Force Plate is selected in the Properties pane.

Live Data

When force plates are synchronized through Gated Internal Clock, the force data may not be available in the Live mode because the signal gets outputted only when Motive is recording. If you need to see the data in Live mode, use the triggered sync setup.

The final frame rate of the camera system will be displayed at the very top of the Devices pane.

In the Devices pane, select the eSync among the listed devices. This will list out the synchronization settings in the Properties pane for the selected eSync.

Set up the output signal so that the recording trigger signal can be sent to the force plate system. In the Outputs section, enable and configure the corresponding output port of the eSync which the force plate system is connected to.

Now that the eSync has been configured, you need to configure the properties of the force plates. While the force plate(s) is selected in Motive, access the Properties pane to view the force plate properties. Here, set the following properties:

Record Trigger → Device

Reference Clock Sync → False

eSync Output Channel → output port used on the eSync.

Once this is done, the force plate system will synchronize to the recording trigger signal when Motive starts collecting data, and the force plates will free-run after the initial sync trigger. You can configure the sampling rate of the force plates by modifying the Multiplier values in Devices pane to sample at a whole multiple of the camera system frame rate.

Example eSync properties for clock sync. Make sure the eSync is selected in the Properties pane.

Example force plate properties for clock sync. Make sure the Force Plate is selected in the Properties pane.

Sampling Rate

Supported Frame Rate: When synchronizing two systems via recording trigger, the force plates will be running at their own free-run frame rate. In this case, only 1000 Hz sampling rate is supported for Bertec force plates. If you wish to sample at a different rate, please use the reference clock sync approach.

Setting Framerate Multiplier: For free run sync setups, sampling rates of force plates can be set from the Devices pane, but the sampling rate of force plates must be configured to a whole multiple of the camera system's framerate. By adjusting the Rate Multiplier values in the Devices pane, sampling rates of the force plates can be modified. First, pick a frame rate of the camera system and then adjust the rate multiplier values to set force plates to the desired sampling rate.

ReSynch

Re-aligning initial sampling timing of the force plate.

When two systems are synchronized by recording trigger signals (Recording Gate or Recording Pulse), both systems are in Free Run Mode. This means that the recording of both the mocap system and the force plate system are triggered simultaneously at the same time and each system runs at its own rate.

Two systems, however, are synchronized at the recording trigger but not by per frame basis. For this reason, alignment of the mocap data and the force plate data may gradually drift from each other for longer captures. But this is not a problem since the sync chain will always be re-synchronized each time recording in Motive is triggered. Furthermore, Takes in general do not last too long for this drift to take effect on the data.

However, this could be an issue when live-streaming the data since recording is never initiated and two systems will be synchronized only when Motive first launches. To zero out the drift, the ReSynch feature can be used. Right-click on force plates from either the Devices pane or the perspective view, and select Resynch from the context menu to realign the sampling timing of both systems.

Before you start recording, you may want to validate that the camera and force plate data are in sync. There are some tests you can do to examine this.

The first method is to record dropping a retroreflective ball/marker onto the platform few times. The bouncing ball produces a sharp transition when it hits the surface of the platform, and it makes the data more obvious for validating the synchronization. Alternately, you can attach a marker on a tip of the foot and step on and off the force plate. Make sure that your toe — closest to the marker — strikes the platform first, otherwise the data will seem off even when it is not. You can then monitor the precise timing of the ball or the foot impacting the force plate and compare them between the mocap data and the force plate data. ↑

The following is an example of validating good synchronization outcomes:

All of the configured device settings, including the calibration, get saved on Device Profile XML files. When you exit out of Motive, updated device profiles will be saved under the program data directory (C:\ProgramData\OptiTrack\Motive\DeviceProfiles), and this file gets loaded again when you restart Motive. The persistent settings folder can be accessed through Help → Application Folders → Persistent Setting. This XML file ensures that all of the device settings are persisted each time you close and restart Motive.

Direct access to the persistent settings folder. In this folder, license files as well as XML files for persistent configurations are stored.

Force plate data can be monitored from the Graph View pane. You will need to configure a custom graph layouts to show force plate data. As shown in the images, make sure the desired force plate data channels (Fx, Fy, Fz, Mx, My, or Mz) are selected to be plotted. Then, when you select a force plate in Motive, and the data from the corresponding channels will be plotted on the graphs. When both reconstructed markers and force plate channels are selected, the force plot will be sub-sampled in order to be plotted along with trajectory data. For more information about how to configure graph layouts, read through the Graph View pane page.

For plotting live force plate data, the graph View Style of the corresponding graph must be configured to Live.

Notes

The force and moment data reflects the coordinate system defined by the force plate manufacturer, which is typically the Z-down right-handed coordinate system. Note: This convention is independent of the global coordinate system used in Motive. Thus, the Fz components represent the vertical force. For more in-depth information, refer to the force plate specifications.

Motive exports tracking data and force plate data into C3D files. Exported C3D files can then be imported into a biomechanics analysis and visualization software for further processing. See the Data Export or Data Export: C3D page for more information about C3D export in Motive. Note that the coordinate system used in Motive (y-up right-handed) may be different from the convention used in the biomechanics analysis software.

Since Motive uses a different coordinate system than the system used in common biomechanics applications, it is necessary to modify the coordinate axis to a compatible convention in the C3D exporter settings. For biomechanics applications using z-up right-handed convention (e.g. Visual3D), the following changes must be made under the custom axis.

X axis in Motive should be configured to positive X

Y axis in Motive should be configured to negative Z

Z axis in Motive should be configured to positive Y.

This will convert the coordinate axis of the exported data so that the x-axis represents the anteroposterior axis (left/right), the y-axis represents the mediolateral axis (front/back), and the z-axis represents the longitudinal axis (up/down).

Force plate data and the tracking data can be exported into CSV files as well. When a Take file is exported into a CSV file. Separate CSV files will be saved for each force plate and it will contain the force, moment, and center of pressure data. Exported CSV file can be imported for analysis.

To stream tracking data along with the force plate data, open the Data Streaming Pane and check the Broadcast Frame Data, and make sure that you are not streaming over the camera network. Read more about streaming from the Data Streaming workflow page.

Motive can stream the tracking data and the force plate data into various applications — including Matlab — using NatNet Streaming protocol. Find more about NatNet streaming from the User's Guide included in the download.

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